A critical problem for developmental neurobiology is to understand the mechanisms by which neurons acquire their specific properties and interconnections. With the exception of a few well-studied systems, little information is available about the exact nature of the intrinsic developmental signals in vivo whereby cells in the central nervous system are progressively restricted to differentiate and elaborate their specific neuronal properties. This lack, in part, represents the difficulty in most systems for examining the developmental history of single, specific, and identifiable cells of known adult properties. The proposed study in the abdominal ganglion of the marine mollusc Aplysia californica will combine morphological, biochemical and physiological techniques to study the nature and consequences for differentiation of 1) cell lineage, 2) neuro-neural interactions and 3) neuro-glial interactions. The ability to raise animals in the laboratory and the beginning already made in studying its development makes Aplysia highly suitable for studying the development of a central ganglion in terms of the program for differentiation of specific identified neurons, their interconnections and their role in behavior.